Makefile: pass CPPFLAGS to the compiler

Distro tools like dpkg-buildpackage use the preprocessor flags to pass
important flags, like hardening features. Pass CPPFLAGS to CC.

Signed-off-by: Luca Boccassi <luca.boccassi@microsoft.com>

src/Makefile

@@ -63,7 +63,7 @@ $(OBJDIR)/libbpf.pc:
 		< libbpf.pc.template > $@
 
 $(OBJDIR)/%.o: %.c
-	$(CC) $(ALL_CFLAGS) -c $< -o $@
+	$(CC) $(ALL_CFLAGS) $(CPPFLAGS) -c $< -o $@
 
 define do_install
 	if [ ! -d '$(DESTDIR)$2' ]; then		\